Begin by chopping the yellow onion and mincing the garlic cloves. Rinse the fresh spinach thoroughly and set aside.
Heat the olive oil in a skillet over medium heat until it shimmers and starts to ripple, about 2 minutes.
Add the chopped onion to the skillet and cook, stirring occasionally, until it becomes translucent and fragrant, about 3-4 minutes. Then, stir in the minced garlic and cook for another 30 seconds until aromatic.
Transfer the sautéed onion and garlic into the slow cooker. Pour in the vegetable broth and sprinkle in the ground nutmeg, stirring to combine.
Add the fresh spinach on top of the broth mixture. It will seem like a lot at first, but it will wilt down significantly during cooking.
Set the slow cooker on low heat and cook uncovered for 4 to 6 hours, until the spinach has wilted completely and the flavors have melded into a fragrant, vibrant green liquid.
Once done, use a ladle to transfer the hot soup into an immersion blender or carefully pour it into a regular blender in batches. Puree until the soup is silky and smooth, about 1-2 minutes.
Return the pureed soup to the slow cooker or a pot on the stove. Stir in the heavy cream if using, and warm gently for another 2-3 minutes until heated through. Taste and adjust seasoning with salt or lemon if desired.
Spoon the warm, velvety soup into bowls, drizzle with a little extra olive oil if you like, and serve immediately for a bright, comforting meal.
